Cemented carbides are used in many cutting applications for metallic and non-metallic materials. Cemented carbides are usually prepared as a sintered product produced from a mixture of tungsten carbide powder and an iron-group binder metal, usually cobalt or nickel. Additives are included in some grades of cemented carbides to obtain improvements in certain properties, such as improved hot strength and resistance to cratering. Such additives may include one or more carbides such as titanium carbide, hafnium carbide, tantalum carbide, niobium carbide, vanadium carbide, molybdenum carbide and chromium carbide, and nitrides such as titanium nitride.
Improved resistance to abrasive wear and cratering without a significant decrease in tool strength has been sought by applying a thin coating on the surface of the cemented carbide. This composite makes it possible to achieve increased resistance to abrasion wear and cratering, provided by the coating, while the substrate has adequate resistance to breakage and deformation. One of the first coatings proposed was titanium carbide which not only substantially improved the life of the cutting tool, but also permitted a considerable increase in cutting speeds. Another coating that has achieved commercial recognition is a so-called ceramic coating in the form of an oxide, such as aluminum oxide.
Chemical vapor deposition (CVD) of metal carbides on cemented carbide substrates has been the subject of investigation in the last two to three decades, as evidenced by U.S. Pat. Nos. 2,962,388 and 2,962,399 issued to Ruppert, and also 3,640,689 issued to Glaski. Thin coatings of nitrides, silicides and carbides of the metals in Groups IVa (Ti, Zr, Hf), Va (V, Nb, Ta), and VIa (Cr, Mo, W) of the Periodic Table of the Elements were applied to cemented carbide substrates for improving the wear characteristics of cemented carbide cutting inserts.
A further development, noted above, has been the addition of a surface layer of a refractory oxide, such as aluminum oxide (Al.sub.2 O.sub.3), or zirconium oxide (ZrO.sub.2), including stabilized zirconium oxide. This development is described in U.S. Pat. Nos. 3,736,107 to Hale and 3,836,392 to Lux.
Multilayer coatings have been developed. These include: U.S. Pat. Nos. 3,955,038 and Re. 29,420 to Lindstrom describing an intermediate layer of carbide or nitride and an outer layer of ceramic oxide; U.S. Pat. No. 4,101,703 to Schintlmeister describing a TiC first layer, TiCN intermediate layer and TiN outer layer; U.S. Pat. No. 4,018,631 to Hale which describes a coating process including the steps of forming a selected metal carbide, nitride or carbonitride coating on a substrate, diffusing tungsten and cobalt from the substrate into the coating, oxidizing the coating, and coating the oxidized coating with an oxide; U.S. Pat. No. 4,357,382 to Lambert describing a TaC/TiC/Al.sub.2 O.sub.3 /TiN coating (noted in the order of deposition of the substrate), with transition layers between the individual coating layers; U.S. Pat. No. 4,442,169 to Graham describing an Al.sub.2 O.sub.3 layer with TiN or TiC deposited on it, with an intermediate TiO layer; and U.S. Pat. No. 4,463,063 to Hale describing an Al.sub.2 O.sub.3 coating with a TiO interlayer provided by reduction of TiO.sub.2.
Reaction-type coatings have also been described. U.S. Pat. No. 4,399,168 to Jullander discloses: first treating the substrate to form a metal carbide, nitride or carbonitride coating; heating the coated substrate near the melting point of the cemented carbide binder metal to diffuse elements from the substrate into the coating; applying an intermediate metal carbide, nitride or carbonitride coating; oxidizing, nitriding or sulfidizing a portion of the second coating; and applying a final Al.sub.2 O.sub.3 coating.
U.S. Pat. No. 4,447,263 to Sugizawa describes a reaction layer of carbonitride (or oxycarbonitride) of at least two group IVa, Va and VIa metals, including titanium.
U.S. Pat. No. 4,490,191 to Hale suggests provision of a thin surface-oxidized bonding layer comprising a carbide or oxycarbide of at least one of tantalum, niobium and vanadium, optionally aluminizing the bonding layer, and finally providing an outer oxide wear layer. This patent to Hale also cites a number of U.S. and Japanese prior art patent references that describe numerous and complex intermediate coating layers.
The wear resistance of titanium oxycarbide sputter deposited coatings on cemented carbide cutting tools is reported in Carson, W. W., C. L. Leung and N. P. Suh, "Metal Oxycarbides as Cutting Tool Materials", Winter Meeting of the ASME, Houston, Tex. 1975, Paper No. 75-WA/Prod. 3. Resistance to flank wear and cratering of TiC.sub.0.75 O.sub.0.25 and TiC.sub.0.5 O.sub.0.5 were found to be comparable to that of TiC sputter coated inserts. However, delamination of the coatings on the flank of the inserts during machining was a persistant problem.
Titanium oxycarbide coatings applied by a CVD method on cemented carbide tools have been described in Kikuchi, J., H. Doi and T. Onishi, "Titanium Oxycarbide Coatings Via CVD Method", Proceedings of the Sixth International Conference on Chemical Vapor Deposition, Ed. by L. F. Donaghey, P. RaiChoudhury and R. N. Tauber, The Electrochem. Soc., Princeton, N.J., 1977. Formation of an intermediate layer of titanium oxycarbide is disclosed also in U.S. Pat. No. 4,463,033 to Kikuchi. From steel turning tests described in the above cited CVD Conference paper, titanium oxycarbide coatings were found to greatly increase cemented carbide tool life. Observations on the growth morphology of Al.sub.2 O.sub.3 particles on a TiC.sub.0.5 O.sub.0.5 coated layer led to the conclusion that this layer is an excellent substrate for an Al.sub.2 O.sub.3 coating.
A CVD process to produce a co-deposited aluminum-titanium-oxide coating on cemented carbide tools is described in U.S. Pat. No. 4,052,530 to Fonzi. The addition of 2 to 10% titanium oxide increased the hardness of the coating to 2400-2500 Knoop. The co-deposited material bonded well to the substrate.
The coatings described by prior art patents have certain features that also are deficiencies. A single layer titanium carbide coating, such as applied by CVD to a cemented carbide substrate, provides substantially improved abrasion resistance, but does not provide nearly as effective resistance to cratering as that provided by aluminum oxide. The TiC layer is single phase TiC, or a single phase TiC-rich solid solution, with small amounts of elements from the substrate and coating atmosphere co-incidently being present.
A two-layer TiC and Al.sub.2 O.sub.3 coating system was developed in an effort to obtain the above-described benefits of each separate coating. In this system, the first coating layer is a TiC phase and the outer layer is aluminum oxide. The aluminum oxide may be present partially as alpha phase Al.sub.2 O.sub.3 and partially as kappa phase Al.sub.2 O.sub.3, as taught in U.S. Pat. No. 4,180,400 to Smith.
In more complex coating systems of the prior art having two or more layers, each layer or lamella is a selected single phase compound (or solid solution phase of that compound). The coating systems, both single layer and multilayer types, require critical manufacturing control of the uniformity and thickness of the individual layers. Such prior art coatings are often prone to cracking, chipping or delamination.
In addition to one or more of the above deficiencies, many prior art coating systems are plagued by the formation of a brittle eta phase at the interface between the first coating layer and the substrate. U.S. Pat. No. 4,150,195 to Tobioka discloses a cemented carbide substrate containing free carbon to provide a coated article free from such eta phase.
It is accordingly an object of the present invention to provide a coated cemented carbide having the benefits of two or more simultaneously co-deposited phases throughout the entire coating, as distinguished from perceived benefits of a single phase within each particular layer as in present coatings.
Another object of the invention is to provide coatings of mixed layers, such as bands of duplex simultaneously co-deposited phases between single layers, to obtain improved toughness and to serve as crack arrestors.
A further object of the invention is to provide a coating system with improved control of the overall coating thickness.
An additional object is to provide a composite coating that minimizes or eliminates formation of eta phase.
Other objects and features of the invention will be found in the following description and claims in which the invention is described along with details directed to those skilled in the pertinent arts of the manner and process of making and using the invention, all in connection with the best mode presently contemplated for the practice of the invention.